Data transmission systems are well known in the communications art. Such systems typically employ a frequency synthesizer circuit (e.g., phase lock loop circuit) for generating a carrier signal, a modulation circuit for modulating the incoming data about the carrier signal, and a power amplifier for boosting the power level of the modulated data signal for transmission. One modulation scheme often employed, frequency shift keying (FSK), provides a modulated signal by altering the frequency of a carrier signal. That is, by changing a predetermined carrier frequency signal by a known amount depending on the value of the incoming data, a signal is provided that can be easily demodulated at the receiving end to reconstruct the original data signal. Thus, FSK is a frequency modulation (FM) technique. It should be noted that the use of phase locked loop (PLL) circuits to generate frequency modulation information on a radio carrier frequency is well known in the art. Such systems are particularly useful in situations where both accurate frequency control and frequency agility are required.
The requirements of accuracy and agility are satisfied by the PLL for signals with an average value of zero, such as a signal produced at the terminals of a microphone. However, in the case of non-return to zero (NRZ) data information, the average value of the signal is not necessarily zero. The PLL will remove the frequency components of the data signal that fall below a particular frequency threshold, which is determined by the construction of the PLL. In principle, this frequency threshold can be made arbitrarily low, but in practice the problems that arise in the physical hardware implementation of a PLL that modulates low frequencies are prohibitive.
A solution to this problem for binary data applications was disclosed in U.S. Pat. No. 4,471,328, entitled "Variable Frequency Reference Source Responsive to Digital Data", incorporated herein by reference. The patent provides, in part, for modification of the reference frequency by adding pulses to, or subtracting pulses from, the signal produced by the fixed frequency source (reference no. 10 in the reference) prior to introduction to a phase locked loop (PLL) circuit. The rate of pulse addition or subtraction is determined by the data signal that is to be modulated (18 in FIG. 1) and the frequency of an oscillator (26 in FIG. 1 ). By adjusting the frequency of the oscillator, the proper FM deviation can be obtained for a given transmitter carrier frequency.
While this technique provides modulation of frequency components down to 0 Hz, it is cumbersome to use because the oscillator frequency must be adjusted for each transmitter carrier frequency and transmitter deviation. Also, to expand the technique to multi-level data signals, e.g., four-level data, would require additional dividers (28 and 30 in FIG. 1) and oscillators (26 in FIG. 1), additional multiplexing circuitry (20, 22, and 24 in FIG. 1) and additional inputs capable of handling multi-level input (note that data input 18 is for binary data exclusively). Therefore, the attendant increase in circuit complexity, cost, and maintenance (i.e., adjustment) becomes prohibitive utilizing the prior art.
Additionally, systems utilizing analog hardware, such as that disclosed in U.S. Pat. No. 4,755,744, entitled "Two Port Synthesizer Modulation System Employing an Improved Reference Phase Modulator", also exist. These systems suffer, however, from the disadvantages of performance variation over temperature, manufacturing tolerance, aging, and other impacts.
Accordingly, there exists a need for a data transmission system transmitter which is not subject to the constraints of the prior art. In particular, a data transmitter that is operable over a variable frequency range without manual adjustment, and is responsive to multi-level data inputs without sensitivity to temperature, aging, and manufacturing tolerance would be an improvement over the existing art. That is, by providing a synchronized, virtually temperature independent modulation scheme, an agile data transmitter would be provided that avoids the short-comings of todays data transmitters.